The proposed research deals with attempts to define and describe the ultrastructure of heart muscle, especially that of the atrium, in several animals including birds, small mammals and the dog. The overall goal is to understand the anatomy of intracellular organelles, cellular geometry and, eventually, the routes through which excitation of heart muscle is effected. In order to do so, the individual cardiac cells will be investigated in detail with electron microscopy taking advantage of phylogenetic and ontogenetic variations as they reveal themselves in different orders and species; morphologic changes during ontogeny and phylogeny may be related to corresponding alterations in function and, thus, may provide insight into the function of various structural components of the cells. In addition to studying individual cells, the proposed research has to do with charting contacts between cells and bundles of especially atrial cells since the overall physiologic behaviour of cardiac muscle, including the atrium, depends on the geometry, as well as other functional parameters of the component cells, e.g. secretory and nerve activity. Emphasis will be on studying the geometry of cells and their apposition in bundles throughout the atrium which will be investigated with the electron microsope after the whole atrium has been embedded in plastic so that topography can be preserved and appreciated.